Development of a medical photoacoustic tomography apparatus (PAT apparatus) using tunable laser is proceeding (NPL 1). Such a PAT apparatus is expected to be capable of determining whether or not a tumor exists by observing blood vessels gathered around the tumor in a living body and analyzing the function of a tissue by making use of a difference in spectrum between oxidized hemoglobin and reduced hemoglobin.
The PAT apparatus is a measuring apparatus configured to acquire an image by irradiating a measurement site by a nanosecond pulse laser, receiving a photoacoustic wave generated therefrom and then analyzing received signals. Because the intensity of laser light applied to a living body attenuates due to diffusion inside the living body, laser light having a high energy output per pulse is needed particularly when a photoacoustic wave has to be obtained from a part located relatively deep inside the living body such as a breast.
A titanium sapphire laser and an alexandrite laser are solid-state lasers each rendered tunable by using a laser medium having a wide gain band. Wavelength selecting systems for allowing a tunable laser to generate laser oscillation of a desired wavelength include one based on an approach such as to provide a prism, a diffraction grating or a birefringent plate in a laser resonator, and one based on an approach such as to utilize an acoustooptic tunable filter (AOTF) (PTL 1).
In cases where the prism is used, a light beam is allowed to resonate in a resonator correspondingly with an angular displacement of optical path which is estimated from the refractive index of the prism. When the prism is used as a reflecting mirror of the resonator, the prism is mechanically rotated in the plane of a substrate forming part of the resonator. When a reflecting mirror is used separately from the prism, wavelength selection is also possible by fixing the prism and mechanically rotating the reflecting mirror located correspondingly with an angular displacement of optical path on a wavelength-by-wavelength basis. In cases where the diffraction grating is used as in the case of the prism, wavelength selection is made by mechanically rotating a reflecting mirror in accordance with an oscillation wavelength.
In cases where the birefringent plate is used, wavelength selection is made by mechanically rotating the birefringent plate in such a manner as to maintain the optical axis of a resonating light beam at the angles of its incidence on and outgoing from the birefringent plate.
In cases where the AOTF is used, wavelength selection is made by means of RF signals applied to the AOTF device without using any mechanically rotating system.